Deconvoluting Cr states in Cr-doped UO2 nuclear fuels via bulk and single crystal spectroscopic studies

Cr-doped UO2 is a leading accident tolerant nuclear fuel where the complexity of Cr chemical states in the bulk material has prevented acquisition of an unequivocal understanding of the redox chemistry and mechanism for incorporation of Cr in the UO2 matrix. To resolve this, we have used electron paramagnetic resonance, high energy resolution fluorescence detection X-ray absorption near energy structure and extended X-ray absorption fine structure spectroscopic measurements to examine Cr-doped UO2 single crystal grains and bulk material. Ambient condition measurements of the single crystal grains, which have been mechanically extracted from bulk material, indicated Cr is incorporated substitutionally for U+4 in the fluorite lattice as Cr+3 with formation of additional oxygen vacancies. Bulk material measurements reveal the complexity of Cr states, where metallic Cr (Cr0) and oxide related Cr+2 and Cr+32O3 were identified and attributed to grain boundary species and precipitates, with concurrent (Cr+3xU+41-x)O2-0.5x lattice matrix incorporation. The deconvolution of chemical states via crystal vs. powder measurements enables the understanding of discrepancies in literature whilst providing valuable direction for safe continued use of Cr-doped UO2 fuels for nuclear energy generation.

ppm Cr2O3 addition sintered at 1700 °C using an oxygen potential of -420 kJ/mol O2.

Supplementary Information Note 3. Single Crystal X-ray Diffraction
Supplementary Table 2 provides the structural solution for the Cr doped UO2 single crystal (SC1) shown in Figure 1 of the manuscript and also measured via EPR, HERFD-XANES and EXAFS (Figure 2a,b and d), indicating the occurrence of the Cr doped UO2 fluorite structure in space group Fm3 ̅ m. No attempts were made to refine the Cr position.
Supplementary Table 3 provides the structural solution for a Cr doped UO2 single crystal (SC2) also examined via HERFD-XANES but found to contain Cr o impurities. The refinement returned larger R factors for SC2 compared to SC1.
Supplementary Table 2. Crystallographic data for a Cr doped UO2 single crystal grain.

Spectroscopy
Supplementary Figure 3 provides measured EPR spectra of Cr2O3 measured at room temperature. The spectra become observable via the lifted antiferromagnetic coupling between Cr +3 cations resulting in g factor of 2.2. Notably the spectra, which is akin to a Cr +3 -Cr +3 cluster, is considerably different to that identified in the Cr-doped UO2 single crystals (Figure 2a).

U M4 and L3 Edge HERFD-XANES
HERFD-XANES measurements were performed on the U M4-edge for the Cr-doped UO2 powder and U L3-edge for the Cr doped UO2 single crystal and powder both with a UO2 standard. The results of these are presented in Supplementary Figure 4. The results show that to limits of resolution the Cr-doped UO2 single crystal and powder contain U +4 identical to that found in a UO2 standard. and sole presence of U +4 . The increased noise in the single crystal grain line spectra is due to its significantly smaller size. Note a.u. denotes arbitrary units. 8

HERFD-XANES Iterative Transformative Factor Analysis (ITFA)
Iterative transformative factor analysis (ITFA) was used to analyse the collected HERFD-XANES spectra and quantify relative amounts of specific Cr chemical states.
Supplementary Figure 5 provides the calculated ITFA components of Cr metal, Cr +2 (in Cr +2 Cl2) and Cr +3 (Cr +3 Cl3·6H2O) compared against measured Cr K-edge HERFD-XANES spectra highlighting the good reproduction of spectra from ITFA analysis.    Figure 2b-c), and another example crystal, SC2, originating from the same bulk material but found to contain a much higher metallic impurity content, HERFD-10 XANES spectra and detailed ITFA analysis results. When the error of the ITFA analysis is considered, a trace surface impurity amount of metallic Cr, Cr 0 , was found to be present in SC1 whereas a greater proportion of metallic Cr was detected in SC2. Comparing their calculated spectra from ITFA analysis to actual measured ones, the pre-edge peak at 5990 eV experiences broadening that is dependent on the amount of metallic Cr present. This effect is illustrated in Supplementary Figure 7. Accordingly, the described broadening effect is due to the presence of metallic Cr. Another notable observation from the additional Cr doped UO2 single crystal grains is that despite present contamination with metallic Cr they appear to be near free of Cr +2 parasitic phases particularly when the known ITFA error is considered. Figure 7. Normalised Cr K-edge HERFD-XANES spectra in the range 5981 to 6001 eV for Cr doped UO2 single crystal grains SC1 and SC2 and metallic Cr, the vertical lines and arrows are guides highlighting the broadening and drift of the pre-edge peak due to variable metallic impurity presence. Note a.u. denotes arbitrary units.